Largest endocrine gland (weighs 20-25gm). ‘Butterfly’ shaped gland in anterior neck, wrapped around trachea, just below larynx. The two lateral lobes connected by the isthmus. Histologically, composed of sacs, called thyroid follicles which store thyroglobulin, from which the hormone is formed.
Thyroid hormone (TH) is a major metabolic hormone with profound effects
Consists of two hormones – thyroxine (T4) and triiodothyronine (T3) – both almost identical in structure, but thyroxine has 4 iodine atoms
Thyroxine secreted by thyroid follicles, triiodothyronine is formed at target tissues by conversion from thyroxine.
Has 3 key actions – stimulates energy use, promotes growth and stimulates the heart (ie increase metabolic rate – calorigenic effect)
Affects almost all body cells (except brain, spleen, testes, uterus and thyroid gland)
Stimulates enzymes associated with glucose oxidation increased metabolic rate and heat production (calorigenic effect)
Secretion is stimulated by TSH.
• produced by parafollicular/C cells of thyroid
• lowers blood calcium levels/antagonist of parathyroid hormone
• inhibits osteoclast activity and stimulates calcium uptake into bone matrix
• stimulated by higher serum level of calcium